1. Field of the Invention
The present invention relates generally to a muffler for a brake booster, and more particularly to a muffler device for use with a brake booster system which comprises, in combination, a cylindrical noise shielding member having a radially outwardly extending flanged portion and a radially inwardly extending flanged portion formed respectively at the front and rear ends thereof, and a cylindrical noise absorbing member having an appropriate air permeability.
2. Description of the Prior Art
The typical construction of a brake booster, which is generally known in the art of pneumatic brake systems is such that a power piston is slidably mounted for reciprocating motion in a housing and is adapted in function to urge a master cylinder piston in the housing. A constant pressure chamber and a variable pressure chamber are formed in front of and behind the power piston, respectively. The power piston is provided with a first valve which is adapted to open and close a passage communicating the constant pressure chamber with the variable pressure chamber and a second valve is adapted to open and close a passage communicating the variable pressure chamber with the atmosphere. These first and second valves are slidably mounted in the inside of the power piston and are adapted to be controlled for opening and closing motion with the forward and reverse motions of a plunger which is operatively connected through an input rod to a brake pedal. More specifically, according to the typical construction of a booster or amplifier mechanism of the vacuum-operated type, the constant pressure chamber as noted above is constantly exposed to a negative pressure, and when the brake pedal is stepped on so as to have the input rod in reciprocating motion, the plunger is caused to move forwardly to close the first valve, and concurrently to open the second valve introducing atmospheric pressure into the variable pressure chamber. This results in a reciprocating motion of the power piston. On the other hand, it is constructed to operate in such a manner that when releasing the brake pedal from its stepped on position, the input rod is forced backward under the urging effort of a return spring so as to have the plunger move in reverse motion, thereby letting the first valve open and at the same time the second valve close so as to dissolve the differential pressure existing between the constant pressure chamber and the variable pressure chamber, and thereby let the power piston moved in returning motion by function of its return spring.
With such a typical construction the brake booster system has a tendency to produce a relatively high level of air intake or suction noise at the variable pressure chamber into which the atmosphere is sucked rapidly from the second valve, when the second valve is opened by stepping down on the brake pedal. This tendency cannot be avoided easily. Also, since the brake booster is located generally in the vicinity of the driver's seat in the vehicle cabin, it is desirable to reduce this noise for prevention of such air suction noises from propagating in and about the environment of the vehicle cabin. In the conventional way of coping with such problem, it was a typical practice to use a certain acoustic material, such as felt texture or the like, as packing for the air suction passage in the inside of a power piston structure in the attempt to somehow absorb such noises from being scattered around in the environment. However, with the use of a too thick acoustic material packings for the air intake passage of the system for the purpose of attaining in haste an effective result of absorbing such air suction noises, there would undoubtedly be produced an undesired effect of resistance against air suction in the air suction passage, which would naturally bring a poor response in operation of the brake booster, and which would eventually result in a very possible delay in the working of the brake when put to use. Such problem would grow still worse in the case of the tandem type brake booster which has a relatively large volume for its variable pressure chamber. A countermeasure for this type of booster has been left unattended accordingly.
The present invention is essentially directed to the provision of an improved muffler device to overcome such inconveniences and difficulties in practice as outlined above and experienced in the use of the conventional muffler for use with the brake booster system.